For what it's worth ...
I've heard anecdotal suggestions about reports (have never seen them) clearly showing that family donations (meaning essentially parent donations) to a given school go up dramatically when more than one child attends that school. That seems intuitive -- if your kids go to different schools, you wonder which school (if any) you should support; often you don't resolve the issue and support neither. Whereas if more than one kid goes to the same school, then it's sort of a family school and the likelihood of donating money is increased.
And then there's the "p-i-s-s-i-n-g off" factor -- if one child attends a school and child #2, assuming child #2 is truly a qualified candidate with a reasonable chance to get in, does NOT get in, then many parents may have a less positive feeling about the school. And this, in turn, might very well affect the willingness to open the wallet.
So, given that we all know colleges are businesses and are absolutely concerned with the financial side of their business, it would just seem logical to me that having a sibling there -- if you are otherwise and independently a quality candidate -- might definitely be a hook (at least at SOME schools).